In typical Cassegrain telescopes for space remote imaging such as space-born imaging satellites, the optical system typically includes a lightweight primary mirror. The intent of the lightweight design is to minimize gravity distortion for ground testing as well as to minimize overall weight of the system. In a recent project, the primary mirror was mounted on six flexure struts to approximate a kinematic mount. The upper and lower end of each strut was joined to a mount pad via a spherical ball joint. Ideally, this spherical ball joint was free to rotate during alignment and testing of the primary mirror and then was locked to the mount pad by injecting an epoxy adhesive into a clearance gap between the strut ball and a retaining cap which threaded onto the mount pad. Problems were encountered in the alignment of the primary mirror because of the weight of the mirror on the strut balls and the friction developed at the interface between the strut balls and the ball sockets when adjusting the struts. The adjustment design required that the lower strut ball, which was threaded to accept the strut, be rotated to adjust the length of the strut and, therefore, the position of the mirror. Residual bending and torsion moments in the strut were present due to friction at the interface and this resulted in an unacceptable distortion of the primary mirror surface. This required a combination of lubrication and off-loading using an external lifting device in order to align the mirror while retaining an acceptable surface quality. However, this is a time-consuming process and, even at the completion of this process, there will likely always be some distortion present.
A "float support" is sometimes used to support the primary mirror during alignment adjustments by providing a continuous or nearly continuous ring which applies an upward force on the back plate of the mirror at a radius which minimizes overall gravity distortion. The mirror is tested for wavefront quality while supported in this manner so that the local forces due to the struts are not present. However, these devices are expensive to design, manufacture and use, and the technique does not eliminate gravity induced error. In the current practice, the wavefront error due to gravity when the mirror is supported on the struts is modeled using a Finite Element code, and then is "backed out" of the measured wavefront yielding the predicted shape of the mirror in the space environment. Thus, any friction in the joints creates mirror shape errors which are not predicted, and would degrade the on-orbit performance if it is not eliminated.
A mirror support device is taught in U.S. Pat. No. 5,035,497 to Itoh. The Itoh device employs a counterweighted lever. The lever is connected to the mirror support member via a series of links and spherical bearings which enable the lever to pivot about the center of gravity of the mirror.
The prior art fails to teach a system for off-loading which allows pivoting of the strut joints without imparting to the mirror any appreciable frictional moment which could cause distortion of the mirror surface.